Final answer:
The phrase 'my sweet' is option A considered informal language, most often used in personal conversations. In contrast, 'sincerely' and 'truly' are formal, typically used in professional settings. Informal language often reflects regional dialects and personal relationships.
Step-by-step explanation:
In determining which of the options provided is an example of informal language, we can analyze each choice within the context of English language conventions. The phrase 'my sweet' is considered informal, often used as a term of endearment in casual conversation or intimate communication, such as between family members or close friends. On the other hand, the words 'sincerely' and 'truly' are more formal, typically found in professional or official correspondence.
Considering the differences in language usage, informal language tends to be more relaxed and indicative of personal relations, while formal language follows the rules of standard English and is often grammatically correct. For example, regional dialects can influence the informality of language - the use of 'y'all' and 'ain't' in Southern American English is a testament to this, despite such forms historically being viewed as informal or incorrect. It is also important to note that, while certain word choices might be informal or nonstandard, users should always ensure they are spelling them correctly and applying them in the appropriate contexts to convey their intended ideas effectively.